How many species of Adenium are there? DNA analysis sheds light on the question

Adeniums are succulent plants native to Africa and the Arabian Peninsula and which are popular ornamental plants. Most botanists recognize 10 or 11 species, but both the taxonomy and nomenclature of this genus are unresolved. In fact, both are a mess. On the taxonomic side, we don’t know how many species there are. Semi-spoiler – there is more than one species, but almost certainly fewer than 10. As for nomenclature, at least two “species” (arabicum and obesum) have invalid names that need to be corrected.

TCSS and CSSA funded a DNA analysis to help resolve these issues. We sequenced five loci (sections of chromosomes) of 43 cultivated Adenium specimens, mostly from known wild localities representing nine morphologically described species. In addition, we tested several additional specimens of unknown or hybrid origin. The results indicate that most of the currently recognized taxa are indeed valid species, while some others are probably not.

This was a preliminary study, and the project is ongoing. The DNA results will be correlated with physical character measurements and geographic distribution data to help settle the questions. Sequencing of more specimens is needed, as well as more field observations. The latter may be a difficult task. The least studied and understood Adenium populations are in countries that are not safe to travel in, such as Somalia, Yemen, and Mali.

Mark A. Dimmitt has a Ph.D. in biology (herpetology) from the University of California at Riverside after earning an M.S. from UCLA and a B.S. from Pomona College. He worked at the Arizona-Sonora Desert Museum from 1979 to 2011, first as Curator of Botany, and eventually as Director of Natural History (field ecologist). His areas of research included botany and vertebrate biology, and he is the author of more than 50 scientific and popular publications about ecology and horticulture. He is a Fellow of the Cactus and Succulent Society of America. His major publication is the plant and ecology chapters of A Natural History of the Sonoran Desert (2000), and is the senior editor of the revised edition (2015).

Mark’s other and ongoing career is as a plant breeder. He spent a couple of decades hybridizing Trichocereus(Echinopsis, cacti), then Tillandsia(bromeliads); he has introduced about 50 cultivars. Since the late 1970s his main focus has been on hybridizing the genus Adenium; ‘Crimson Star’, ‘Evelyn Marie’, and ‘Bouquet’ are among his creations. He is coauthor of the book Adenium: Sculptural Elegance, Floral Extravagance (2008). Mark also collects and grows a number of other weird plants, mostly succulents and epiphytes.